Adsorption processes for the separation of oxygen and nitrogen from air are being increasingly used for commercial purposes for the last two decades. Presently, 4-5% of the world's oxygen demand is met by adsorptive separation of air. However, the maximum attainable oxygen purity by-adsorption processes is around 95%. Separation of 0.934 mole percent argon present in the air from oxygen being a limiting factor to achieve 100% oxygen purity by adsorption methods. However, there are many situations where high purity oxygen (&gt;99%) is desired. For example, the efficiency of welding and cutting processes using oxygen is greatly dependent upon the purity of oxygen available. For these applications, purity of at least 99.5% oxygen is customarily specified. Furthermore, oxygen-argon separation is also needed for purification of argon produced during cryogenic separation of oxygen and nitrogen from air. The crude argon (95-97%) produced in such processes will have nitrogen and oxygen and is required to be further purified. Presently high purity argon (99.999%) is produced by catalytic hydrogen combustion or low temperature oxygen adsorption in a synthetic zeolite.
Oxygen and argon gaseous mixture is difficult to separate due to closeness in their physical properties. At present, commercially this is done by cryogenic fractionation techniques. The boiling points of oxygen (-182.97.degree.), argon (-185.9.degree. C.) and nitrogen (-195.8.degree. C.) being very low make these processes highly energy intensive. Thus, it is desired to develop a commercially attractive separation process for oxygen-argon separation. Adsorption based process can compete with highly energy intensive cryogenic fractionation of oxygen/argon mixture if a suitable adsorbent which is selective towards one of the components and which possesses requisite adsorption capacity is commercially available.
In the prior art, adsorbents which are selective for argon from its mixture with oxygen has been reported (PCT Int. Appl. 94. 06. 541. Mar. 31, 1994) by impregnation of silver in commercial zeolites. However, the adsorption selectivity reported for argon is less than 2 in these adsorbents making it commercially unattractive. Oxygen with purity &gt;99% has been produced (U.S. Pat. No. 4,813,979, 1989) by using carbon molecular sieve adsorbent in which argon is selectively adsorbed due to its smaller kinetic diameter of 3.40A.degree. compared to 3.46A.degree. of oxygen. However, there are no reports on the development of adsorbent which is selective towards oxygen from its mixture with argon in the literature. The present invention deals with the development of synthetic zeolite based oxygen selective adsorbents which can for the first time separate oxygen from a gaseous mixture of oxygen and argon.
Adsorption processes are also being used on a commercial scale for the production of nitrogen from air. These processes employ carbon molecular sieve type adsorbents in which oxygen diffuses faster than nitrogen resulting in the separation of the two components. Some efforts to develop zeolite type adsorbents for these applications have also been reported in the literature wherein the differences in the diffusion of oxygen and nitrogen have been used to achieve oxygen adsorption selectivity. It is desired to develop a zeolite based adsorbent which can result in the oxygen adsorption selectivity due to difference in equilibrium adsorption of oxygen and air.
The characteristics which are highly desirable, if not absolutely essential, for an adsorbent to be suitable for selective adsorption process include adsorption capacity of the adsorbent and adsorption selectivity for a particular component.
Adsorption capacity of the adsorbent is defined as the amount in terms of volume or weight of the desired component adsorbed per unit volume or weight of the adsorbent. The higher the adsorbent's capacity for adsorbing the desired component the better the adsorbent is as the increased adsorption capacity of a particular adsorbent helps to reduce the amount of adsorbent required to separate a specific amount of a component from a mixture of particular concentration. Such a reduction in adsorbent quantity in a specific adsorption process brings down the cost of a separation process.
Adsorption selectivity of component A over B is defined as EQU O.sub.AB =X.sub.A Y.sub.B /Y.sub.A X.sub.B
where O is adsorption selectivity, X is the adsorbed concentration and Y is gas-phase concentration. The expression gas-phase concentration means the amount of unadsorbed component remaining in the gas-phase. The adsorption selectivity of a component depends on
steric factors such as difference in the shape and see of the adsorbate molecules; PA1 equilibrium effect, i.e., when the adsorption isotherms of the components of the gas mixture differ appreciably; PA1 kinetic effect, when the components have substantially different adsorption rates.
It is generally observed that for a process to be commercially economical, the minimum acceptable adsorption selectivity for the desired component is about 3 and when an adsorption selectivity is less than 2, it is difficult to design an efficient separation process.
Zeolites which are microporous crystalline aluminosilicates are finding increased applications as adsorbents for separating mixtures of closely related compounds. Zeolites have a three dimensional network of basic structural units consisting of SiO.sub.4 and AlO.sub.4 tetrahedral linked to each other by sharing of apical oxygen atoms. Silicon and aluminum atoms lie at the center of the tetrahedral. The resulting aluminosilicate structure which is generally highly porous possesses three dimensional pores the access to which is through molecular sized windows. In a hydrated form, the preferred zeolites are generally represented by the following Formula [I] EQU M.sub.2/n O:Al.sub.2 O.sub.3 :xSiO.sub.2 :wH.sub.2 O [I]
where "M" is a cation which balances the electrovalence of the tetrahedral and is generally referred to as extra framework exchangeable cation, n represents the valency of the cation, x and w represent the moles of SiO.sub.2 and water respectively. The cations may be any one of the number of cations which will hereinafter be described in detail.
The attributes which make them attractive for separation include, an unusually high thermal and hydrothermal stability, uniform pore structure, easy pore aperture modification and substantial adsorption capacity even at low adsorbate pressures. Furthermore, zeolites can be produced synthetically under relatively moderate hydrothermal conditions.
Zeolite of type X structure as described and defined in U.S. Pat. No. 2,882,244 are the preferred adsorbents for adsorption separation of the gaseous mixture described in this invention. Zeolite of type X in hydrated or partially hydrated form can be described in terms of metal oxide of Formula II EQU (0.9+/-0.2)M.sub.2/n O:Al.sub.2 O.sub.3 :(2.5+/-0.5)SiO.sub.2 :wH.sub.2 O[II]
where "M" represents at least one cation having valence n, w represents the number of moles of water the value of which depends on the degree of hydration of the zeolite. Normally, the zeolite when synthesized has sodium as exchangeable cations.
Zeolites as such have very little cohesion and it is, therefore, necessary to use appropriate binders to produce the adsorbent in the form of particles such as extrudates, aggregates, spheres or granules to suit commercial applications. Zeolitic content of the adsorbent particle vary from 60 wt % to 100 wt % depending on the type of binder used. Clays such as bentonite, kaolin and attapulgite are normally used as inorganic binders for agglomeration of zeolite powders.